`Checker.is_final_enum_value` duplicates `TypoInfo.enum_members` logic

[sobolevn]

After merging #18559 I think it would make sense to change how we check for @final enums here:

mypy/mypy/checker.py

Lines 2705 to 2727 in d4e7a81

	def is_final_enum_value(self, sym: SymbolTableNode) -> bool:
	if isinstance(sym.node, (FuncBase, Decorator)):
	return False # A method is fine
	if not isinstance(sym.node, Var):
	return True # Can be a class or anything else
	# Now, only `Var` is left, we need to check:
	# 1. Private name like in `__prop = 1`
	# 2. Dunder name like `__hash__ = some_hasher`
	# 3. Sunder name like `_order_ = 'a, b, c'`
	# 4. If it is a method / descriptor like in `method = classmethod(func)`
	if (
	is_private(sym.node.name)
	or is_dunder(sym.node.name)
	or is_sunder(sym.node.name)
	# TODO: make sure that `x = @class/staticmethod(func)`
	# and `x = property(prop)` both work correctly.
	# Now they are incorrectly counted as enum members.
	or isinstance(get_proper_type(sym.node.type), FunctionLike)
	):
	return False
	return self.is_stub or sym.node.has_explicit_value

There's no need to duplicate the .enum_member logic here once again. I will refactor this piece.

[sobolevn]

This cause a problem: now member and nonmember types are supported in .enum_members, but not in is_final_enum_value. As the result, this code does not work properly:

from enum import Enum, nonmember

class My(Enum):
    x = nonmember(1)
    
class Other(My):  # E: Cannot extend enum with existing members: "My"
    y = 2

While in runtime it works just fine.

Solving it!